For more than three decades, physicists have been looking for, well, next to nothingelementary particles of matter called electron-neutrinos that have no charge and practically no mass. Nuclear reactions that fuel the Sun should churn out vast quantities of these particles, and yet previous measurements have indicated that only about half the number predicted ever reach Earth. Now, American, British and Canadian scientists working at the Sudbury Neutrino Observatory (SNO) in Ontario, Canada say they've discovered what happens to the missing neutrinos. They presented their results yesterday at a conference of the Canadian Association of Physicists and in a paper submitted to Physical Review Letters.

The researchers say that the additional neutrinos are not in fact missing, but rather transformed during their 93-million-mile journey to Earth into two other types of neutrinosmuon-neutrinos and tau-neutrinos. The Sun does not possess enough energy to produce these two types. To measure the number of electron-neutrinos reaching Earth, the SNO team monitored miniscule flashes of light produced when the particles interact with molecules of heavy waterin which deuterium atoms replace the hydrogen atoms. SNO did not detect the muon- and tau-neutrinos because they cannot break up heavy water molecules.

The researchers then compared the number of neutrinos they counted with an experiment run at the Super-Kamiokande detector in Japan. The Japanese test used regular water and thus measured muon- and tau-neutrinos, as well as electron-neutrinos. Because the Japanese detector counted more neutrinos than the SNO detector did, the researchers determined that solar electron-neutrinos must transform to the other types on their way to Earth.

"We now have high confidence that the discrepancy is not caused by problems with the models of the Sun but by changes in the neutrinos themselves as they travel from the core of the Sun to the Earth," says Art McDonald, a professor of physics at Queen's University in Kingston, Ontario and one of the directors of the SNO project. The researchers also determined that neutrinos definitely have mass, albeit a tiny amount. But according to Hamish Robertson of the University of Washington, who worked on the project, even though there are an enormous number of neutrinos in the Universe, they make up only a small fraction of its total mass content.